Photosynthesis is a reaction for converting carbon dioxide and water into oxygen and starch by using solar light as energy. Such photosynthesis actively occurs in green plants and photosynthesis bacteria. Contrary to the photosynthesis in the realm of nature, artificial photosynthesis is a reaction for converting carbon dioxide and water into oxygen, liquid fuel, etc., by using solar light as energy.
In the chemical aspect, carbon dioxide (CO2) and water (H2O) are materials having very low potential energy, while fuel and oxygen are materials having relatively high potential energy. Plants in the realm of nature convert carbon dioxide and water into carbohydrate and oxygen (O2) by using solar energy through photosynthesis. Once the converted carbohydrate and oxygen are reacted once again, they are converted back into carbon dioxide and water. Heat equal to the difference in potential energy between the two reaction materials is discharged outward.
Meanwhile, human beings have obtained energy through combustion of fossil fuel, that is, a reaction for reacting fossil fuel with oxygen to produce carbon dioxide and water. As a result, a concentration of carbon dioxide in the air is increasing every day. This has become a primary cause for global warming. The global warming is being faster and has been regarded as one of the critical factors of the global environmental problems. Accordingly, there have been worldwide efforts to raise applicability of renewable energy such as solar energy, water power, wind power, tidal energy, geothermal heat, bio fuel, etc., instead of the fossil fuel. Among the energy, solar energy is the most promising renewable energy.
As a conventionally developed method for utilizing solar energy, there is a method for converting solar heat and solar light into electric energy. However, of the electric power produced throughout the world, the electric power produced by using solar energy is too small in an amount to be ignored. Moreover, a solar cell has already reached the limit of its efficiency, and the unit price of production of solar cells is increasing. Accordingly, the necessity to realize artificial photosynthesis for producing useful materials by using solar light, water and carbon dioxide is increasing.
However, the artificial photosynthesis has not been realized even at the level of a laboratory despite the fact that many scientists have put forth their efforts for the last century. For example, Japanese Patent Publication No. 2009-034654 describes a method for producing a methane gas by reacting carbon monoxide, carbon dioxide, or a mixture gas thereof with a hydrogen gas under the presence of an iron group transition element, but is disadvantageous in that a yield of the methane gas is poor. In addition, the same Japanese patent publication does not recognize a specific apparatus for realizing photosynthesis. Accordingly, success in researching an apparatus for realizing the artificial photosynthesis is expected to greatly contribute to improvement of global environment and development of scientific technologies.